RC8 Collimation issues

[cardconvict]

Can anyone look at my image from last night i think I'm having issues with collimation but not sure.

[VilleM]

All of your stars are streched slightly. IMHO looks like slight amount of tracking error.

[Big Bang!]

How long was the exposure?   I think a tracking error would have to be huge to show up in a frame under 20 seconds.   There looks like an issue in the top left corner and there is a dark ring on the inside of the right half that is not on the left half so maybe "push" the bottom right and "pull" the bottom left?  When you de-focus the stars I think you may find it better to not go so far out.  A bright ring (rather than doughnut) makes it easier to see where the bright and dark spots are. 

(I have been trying to collimate my RC250TT for over 2 months and it is driving me crazy. The laser seems to be showing the Secondary to be spot on.  When I adjust the Primary to get round stars the laser shows the Secondary to be out of alignment again.  I just cannot figure it out.)

 

Gus

[Big Bang!]

Sorry, I can now see the exposure in the SGP window.  0.4 is very short - a tracking error would have to be enormous to show up in that length.  Really don't think it is tracking.  

 

Gus

[cardconvict]

The single sub is a ten minute exposer guided and was guiding perfectly all night before the clouds rolled in.

[vlaiv]

I don't believe it is collimation issue. But to make sure, examine star image with much less defocus than you showed here.

From elongation orientation I would suggest it is mostly due to RA error, and maybe a bit of PA error - It looks like high frequency periodic error usually caused by gear tooth period (order of 20s) that can have amplitude of 1-2". I'm battling that problem at the moment as well since my target resolution is similar (and I plan exactly the same setup as yours but on heq5 in near future).

If I'm not mistaken you were imaging at 0.86"/pixel - RC8 + x0.67 reducer and ICX694AL chip (4.54um pixel).

If you used 9x50 Finder and qhy5Lii to guide - that would give you guiding resolution of around 4.3"/pixel - so you were at the bounds of resolution for guiding (x5 imaging resolution), but you need to know how your mount behaves when guiding.

Did you note and remember guide RMS errors for RA and DEC? Do you know your RA peak2peak error when guiding? You mentioned perfect guiding. For this resolution I would say that you need to be below 0.5" RMS, and avoid significant peak errors. Having RA RMS over 1-1.5" and DEC below 0.8" can have such impact on star shape.

Also you are using reducer, so make sure you have right working distance and there is no tilt in camera or something like that.

 

[johnrt]

Doesn't look like collimation to me. Have you checked that the primary is secure in it's cell? I had this issue and it drove me mad for months until I worked out what it was. 

[cardconvict]

24 minutes ago, vlaiv said:

I don't believe it is collimation issue. But to make sure, examine star image with much less defocus than you showed here.

From elongation orientation I would suggest it is mostly due to RA error, and maybe a bit of PA error - It looks like high frequency periodic error usually caused by gear tooth period (order of 20s) that can have amplitude of 1-2". I'm battling that problem at the moment as well since my target resolution is similar (and I plan exactly the same setup as yours but on heq5 in near future).

If I'm not mistaken you were imaging at 0.86"/pixel - RC8 + x0.67 reducer and ICX694AL chip (4.54um pixel).

If you used 9x50 Finder and qhy5Lii to guide - that would give you guiding resolution of around 4.3"/pixel - so you were at the bounds of resolution for guiding (x5 imaging resolution), but you need to know how your mount behaves when guiding.

Did you note and remember guide RMS errors for RA and DEC? Do you know your RA peak2peak error when guiding? You mentioned perfect guiding. For this resolution I would say that you need to be below 0.5" RMS, and avoid significant peak errors. Having RA RMS over 1-1.5" and DEC below 0.8" can have such impact on star shape.

Also you are using reducer, so make sure you have right working distance and there is no tilt in camera or something like that.

 

Thanks for replying, I was using a ST80 on the QHY5L ii I'm out again tonight and i will late post the results of PHD and images taken

[cardconvict]

One thing I have noticed in PHD is when i have finished calibrating I turn on guiding assistant the DEC stays around the middle part of the graph with about 1.9 polar alignment error but the RA goes down and straight of the graph and the star does drift away ?

[vlaiv]

If you use ST80 then it certainly have a good guiding resolution, as for RA going of the graph - that is either periodic error (most likely), you might be running at different speed then sidereal (check if you have selected moon or sun tracking rate by accident or on purpose and forgot to switch to sidereal), you might have wrong PEC turned on (EQMOD simply remembers last PEC file and plays it by default, so if you played with PEC before it might have stayed in EQMOD and if there is old one make sure you turn of the PEC), or there is mechanical stall in the system.

It is not unusual for PE to have 20", 30" even 40" peak to peak error on these (synta) mounts. Did you do PE analysis for your mount? Do you know its characteristic? Most of the time when imaging above 1.5"/pixel these mounts work ok but when going below 1"/pixel things start to show so you have to make sure you understand how your mount behaves and what is the best way to image/guide on those resolutions.

[cardconvict]

I have not been using eqmod for long but I have set the sidereal on the correct setting. Not used the PEC program either. The only thing I have done in the past was a strip down of the mount and serviced the mount and maybe the RA worm gear is to right causing it to slow down possibly ?

[cardconvict]

Ok I do have a issue with the mount in RA running slower but i have solved the issue with the collimation as the egged shaped stars was caused by collimation. I used CCD inspector to collimate the secondary mirror and when i went to take my next set of images it was bang on.

 

 

frame_and_focus_1.TIFF

M27_600sec_1x1_R_frame1.TIFF

[vlaiv]

It might be the reason.

Do you have guide log from that session? A lot can be found out by analyzing the phd2 log file - like RA periodic error profile, is there any kind of periodic jolts (I had such case, and vibration also on my mount due to cracked housing on one of RA worm shaft bearings), and is RA leading or trailing in regards to earth rotation.

[vlaiv]

1 minute ago, cardconvict said:

Ok I do have a issue with the mount in RA running slower but i have solved the issue with the collimation as the egged shaped stars was caused by collimation. I used CCD inspector to collimate the secondary mirror and when i went to take my next set of images it was bang on.

M27_600sec_1x1_R_frame1.TIFF

Yes, that looks much better. Glad you sorted it out.

[cardconvict]

Here are the PHD logs from last night

PHD2_GuideLog_2016-06-04_215853.txt

PHD2_GuideLog_2016-06-05_221023.txt

PHD2_GuideLog_2016-06-06_014622.txt

PHD2_GuideLog_2016-06-06_020849.txt

[cardconvict]

40 minutes ago, vlaiv said:

It might be the reason.

Do you have guide log from that session? A lot can be found out by analyzing the phd2 log file - like RA periodic error profile, is there any kind of periodic jolts (I had such case, and vibration also on my mount due to cracked housing on one of RA worm shaft bearings), and is RA leading or trailing in regards to earth rotation.

The RA is trailing to the earth's rotation

[vlaiv]

From what I can see in the logs and I'm by no means expert in this, distorted stars might be the consequence of guiding that is not quite there yet for the resolution you are working with. Most of the time your combined RMS is >1.5" and it should ideally be <0.5" but if you can get it to be <0.8" that would be great. There are excessive peaks also >3.0" - that is not good.

Graphs show that there is no repetitive jolts that are distinct, so I don't believe there is issue with overtightening but you might as well check that again. What I would suggest you to do is - if you are going to open your mount for checking, take a look at all the bearings in the mount and feel them out in your hand, they should run really smooth, if at any point feel like when you roll a wheel over sand on strong surface (something like when you roll a bottle over the table and the table is not clean - you will feel it), or they don't feel the same all the way thru the rotation - they should be replaced. There is a lot of jitter in the graphs from log files and it might be due to bearings.

 

 

[cardconvict]

Ok thanks for looking I will strip down today and have a look. Just want to say as well thank you so much for helping

[Big Bang!]

11 hours ago, cardconvict said:

I used CCD inspector to collimate the secondary mirror and when i went to take my next set of images it was bang on.

Well done in solving that! 

I would be interested to know what CCD Inspector told you to do to the Secondary - which way to adjust it etc. and is there are rule of thumb by which to remember it?

Gus

[cardconvict]

Thanks Gus I followed this guide here https://daveandtelescope.wordpress.com/2013/12/24/collimating-the-at10rc/comment-page-1/#comment-56

[tony210]

Beware focuser alignment issues with these scope - I have a 12inch RCT now and had to adjust the focuser to get round stars- rotating the camera will reveal if the focuser is an issue - may not be the cause here  but it is a common inherent problem with these scopes - best wishes Tony

[cardconvict]

13 hours ago, tony210 said:

Beware focuser alignment issues with these scope - I have a 12inch RCT now and had to adjust the focuser to get round stars- rotating the camera will reveal if the focuser is an issue - may not be the cause here  but it is a common inherent problem with these scopes - best wishes Tony

Thanks Tony

 

I went out the next night and the stars where elongated again. When you say turn the focuser does the elonation turn with the camera as well ?

[tony210]

No- if the focuser is not orthogonal with the back plate of the scope the elongation axis should  change as you rotate the camera but leave the focuser be in order to establish this - best wishes Tony

[kookoo_gr]

Things that might confuse you with the collimation of the RC. I found them out the hard way

1. Polar alignment will affect the image quality and think you have bad collimation

2. Bad focus, make sure you have perfect focus since bad focus will show elongated stars, use a bahtinov mask or a software based focusing system

3. Bad guiding

4. Periodic error of the mount. I believe thsi is the most important that also affects guiding. At my scope  always had an elongation even though i was (and still am) paranoid about collimation and i always had that issue. I finally modded my mount with the Rowan belt mod and the guiding error fell under 1.0 and i usually have it at 0.8''-0.5'' with an OAG. This is also confirmed with another fellow amateur astronomer with the use of VCL200, he had issues with the collimation which finally proved to be errors in the gears after he belt moded the mount and does not have that issue.

5. A sturdy mount tripod or pier will negate any vibrations that might translate into collimation/guiding errors

6. Remove the scope's cone error

 

One way to quickly check the collimation of your scope is to use a semi-bright star and a bahtinov mask. Focus the star at the center of your FOV and don't change it (a word of caution with this is that if the scope is not temperature balanced with the environment the focus will change and you will not have correct results, happened to me a few weeks ago). Move the star to each corner of the FOV, using the bahtinov mask rotate the mask 90 degrees at each corner and observe the focus of the star, you should have similar focus paterns at each corner.

[cardconvict]

Thanks for the advice, Sorry I meant when i rotated the camera the elongation changed.